The so-called investment casting process is simply to make a fusible model with fusible materials (such as wax or plastics) and coat it with some special refractory coatings. After drying and hardening, the model is melted out from the shell by steam or hot water. Then the shell is placed in a sand box, filled with dry sand around it, and finally the model is made by dry sand. Mold is put into the calciner and roasted at high temperature (for example, when high strength shell is used, the shell after demoulding can be directly roasted instead of moulding). Molten metal is poured into the mould or shell after roasting to obtain the casting.
The dimension accuracy of investment casting is high, generally up to CT4-6 (sand casting is CT10-13, die casting is CT5-7). Of course, due to the complex process of investment casting, there are many factors affecting the dimension accuracy of casting, such as shrinkage of mould material, deformation of investment mould, linear change of shell during heating and cooling, shrinkage rate of alloy and deformation of casting during solidification. Although the dimensional accuracy of through-melt casting is high, its consistency still needs to be improved (the dimensional consistency of castings with medium and high temperature wax is much higher).
When pressing the melting die, the high surface finish of the cavity is used, so the surface finish of the melting die is also higher. In addition, the shell is made of a refractory coating made of a special high temperature binder and refractory material, which is coated on the melting die. The inner surface of the cavity directly contacted with the molten metal has a high smoothness. Therefore, the surface finish of investment castings is higher than that of ordinary castings, generally up to Ra.1.6-3.2 um.